To a sheet for thermal transcription, a dye containing a sublimable dispersion dye is thermally transferred from a thermal transcription sheet, by a thermal head, and an image is formed on the sheet for thermal transcription by the so transferred dye. On the thermal transcription sheet, there are provided yellow, magenta and cyan dyes for each image, followed by a laminate film for protecting the image, for extending in a row along the running direction. An image is formed by thermal transcription of yellow, magenta and cyan, on the sheet for thermal transcription, and finally the laminate film is thermally transferred to the so formed image.
The sheet for thermal transcription includes a sheet-like substrate, and a receiving layer formed on the substrate to receive thermally transferred dyes (see Patent Publications 1 and 2, for example). The substrate is a film of plastics, such as, for example, polyethylene terephthalate (PET), polypropylene (PP) or polyethylene (PE), or a sheet of synthetic paper, coat paper, art paper or cast coat paper. The film or the sheet may be used alone or a plural number of the films or sheets may be stuck together (see Patent publication 3, for example).
The receiving layer formed on the substrate receives the dyes transferred from the thermal transcription sheet to hold the so received dyes. The receiving layer is formed of a dyeable resin, such as acrylic resins, polyesters, polycarbonates or polyvinyl chloride.
The receiving layer, composed of the dyeable resin, is added by e.g. polyisocyanate, as a curing agent, and also as a thermal resistance improver. The receiving layer is also added by a plasticizer for improving the transfer sensitivity of the dye and suppressing the fading, that is, for improving light fastness. A silicone oil, for example, is added as mold release agent to the receiving layer for improving its detachment performance.
The receiving layer of the sheet for thermal transcription is required to allow for good running performance and image saving performance, under high temperature conditions, at the same time as it allows for high printing density, light fastness and good transfer performance for the laminate film adapted for protecting the transcribed dyes.
If only the aforementioned acrylic resin is used as a resin of the receiving layer, the transfer performance of the laminate film and the detachment performance of the thermal transcription sheet are optimum. However, in this case, the dye is not optimum in dyeability such that it is difficult to obtain a satisfactory printing density. Also, if only the aforementioned acrylic resin is used as a resin of the receiving layer, there are cases where the response to an external stress is poor and cracks tend to be produced on bending the sheet for thermal transcription.
On the other hand, if only the aforementioned polyester is used as the resin of the receiving layer, sufficient printing density may be obtained because of high dyeability of the dye. However, in this case, the amount of the functional groups reacting with the curing agent becomes extremely small, so that it becomes necessary to add an excess amount of the curing agent to generate cross-linking in the receiving layer to allow for satisfactory running performance under high temperature conditions. However, if the curing agent is added excessively, the printing density or the light fastness is lowered to deteriorate transfer properties of the laminate film.
If the acrylic resin is used as a resin of the receiving layer, there are cases where the amount of addition of the curing agent is decreased and the plasticizer is added to lower the glass transition temperature to provide for excess softening of the receiving layer such as to improve the printing density. If the receiving layer is softened excessively, the printing density is optimum and the dye is diffused sufficiently to improve light fastness and transfer properties of the laminate film. However, if, in this case, the image is stored under high temperature conditions, the dye is diffused in the in-plane direction as well to cause bleeding in the image. Moreover, if the receiving layer is softened excessively, the receiving layer is fused to the dye surface of the thermal transcription sheet, thus lowering the detachment performance of the thermal transcription sheet. If the thermal transcription sheet is lowered in the detachment performance, the image formed may be deteriorated in dignity or defects such as running troubles may be produced.
Since the thermal transcription sheet is heated to higher temperatures for increasing the speed of thermal transcription, high heat is supplied to the sheet for thermal transcription, thus further lowering the detachment performance of the thermal transcription sheet to cause defects such as running troubles.
Thus, in the sheet for thermal transcription, there are cases where the amount of addition to the receiving layer of polyisocyanate as the curing agent is increased to cause excessive curing of the receiving layer in order to improve the running performance or thermal resistance under high temperature conditions and in order to prevent the image bleeding. However, if the receiving layer is cured excessively, the transfer sensitivity and the printing density are lowered severely. If the receiving layer is cured excessively, the receiving layer is not softened with heat generated at the time of thermal transfer, thus causing troubles in transferring the laminate film. Moreover, if the receiving layer is cured excessively, it may occur that the dye is not diffused sufficiently and light fastness is lowered.
If the resin in the sheet for thermal transcription comprises only polyester, and the dyes of yellow, magenta and cyan are sequentially overlaid and thermally transferred, the dye already transferred to the receiving layer may be prevented from being moved back to the thermal transcription sheet. However, if only the polyester is used as the resin, light fastness is poor, and the upper layer dye of the layered dyes tend to undergo fading, thus deteriorating the image.
Thus, it is difficult to provide such a receiving layer of the sheet for thermal transcription which will assure high printing density, high adhesion performance of the laminate film and high running performance, prevent the image bleeding or fading, and which will yield an image of high quality and high resolution, all the more so in case printing is carried out under high temperature conditions, as in the case of high speed transcription.
Patent Publication 1JP Laid-Open Patent Publication H7-117371Patent Publication 2JP Laid-Open Patent Publication H7-68948Patent Publication 3JP Laid-Open Patent Publication H9-267571